There are the 14 known mammalian isoforms of carbonic anhydrases. Each of these isozymes possesses carbon dioxide-hydrating activity that functions to maintain intracellular pH. Carbonic anhydrase isozyme III (CAIII) is distinguished from the other isoforms by several characteristics--particularly, by a low specific activity that is about 1 per cent that of isozyme II. CAIII is not inhibited by acetozolamide, an excellent inhibitor of CAI and CAII. Also, the amount of CAIII is remarkably high in muscle tissue and adipocytes, constituting about 8 and 25 per cent of the soluble protein content of these tissues. Despite numerous publications on the enzyme, the physiological function of CAIII remains unknown, although it has been suggested to be important during aging. We created a knockout to facilitate investigation of the function of CAIII. Offspring of heterozygote matings were genotyped. A large cohort of wild-type, heterozygote, and knockout animals was bred, housed under standard animal husbandry conditions, and followed until natural death in order to determine alterations in phenotype. None were observed. Microarray analyses have now been completed to identify genes whose mRNA levels changed in the knockout; the pattern of changes may provide insight into the function of CAIII.